1. Field of the Invention
The present invention relates to a viewfinder optical system used in a photographic camera or in a video camera.
2. Description of the Prior Art
A camera of a type having a viewfinder optical system independently of a photographic optical system has been known. In this type of camera, the viewfinder optical system must alter its magnification if a zoom lens system is used as the photographic optical system. Therefore, a plurality of kinds of variable magnification viewfinder optical systems have been proposed.
Japanese Laid-Open Patent Application No. 61-160712 proposes a variable magnification viewfinder optical system of inverted Galilean type. The variable magnification viewfinder optical system, proposed by Japanese Laid-Open Patent Application No. 61-160712, comprises from the object side, an objective lens group of a negative refractive power including a positive front lens unit and a negative rear lens unit, and an eyepiece lens group of a positive refractive power, wherein the positive front lens unit and the negative rear lens unit are independently shiftable along the optical axis of the optical system for continuously altering magnification thereof while the eyepiece lens group is stationary on the optical axis. Additionally, Japanese Laid-Open Patent Application No. 61-160712 also proposes, in its embodiments, a combination of an inverted Galilean type finder lens system and an Albada finder system. Therefore, a virtual image of a photographic frame is formed by the Albada finger system together with a virtual finder image formed by the inverted Galilean type finder lens system. Namely, in some embodiments disclosed in Japanese Laid-Open Patent application No. 61-160712, a half-mirror layer is provided on a rear surface of the negative rear lens unit of the objective lens group for reflecting light coming from a photographic frame located behind the objective lens group.
However, the above-explained type finder optical system, which forms the virtual finder image together with the virtual image of the photographic frame, has drawbacks in which a finder field covered by the virtual image of the photographic frame may be changed in accordance with the change in position of the observer's eye, and in that the virtual image of the photographic frame may be blurred in altering the finder magnification. It should be also necessary to design the optical system so as to prevent the occurrence of any ghost and flare due to the half-mirror. Additionally, the diameter of the objective lens group may have to become large for obtaining more change in the finder magnification.
On the other hand, the other type viewfinder optical system has also been known in which a real image of an object is formed by an objective lens group of a positive refractive power and a photographic frame is located at the position on which the real image of the object is formed by the objective lens group, and in which the real image and the photographic frame are observed through an eyepiece lens group located at the rear side of the photographic frame. In this type of viewfinder optical system, the photographic frame can be observed clearly and a correct finder field can be indicated by the photographic frame even if the position of the observer's eye is changed.
Japanese Laid-Open Patent Application No. 61-156018 proposes a variable magnification viewfinder optical system comprising, from the object side, an objective lens group of a positive refractive power including a first negative lens unit, a second positive lens unit and a third positive lens unit, and an eyepiece lens group of a positive refractive power, wherein a real image of an object is formed nearby the third lens unit by the first and second lens units, and wherein the first and second lens units are shiftable for continuously altering the finder magnification.
However, the variable magnification viewfinder optical system proposed by Japanese Laid-Open Patent Application No. 61-156018 has an insufficient change of the finder magnification. Additionally, the variable magnification viewfinder optical system proposed by Japanese Laid-Open Patent Application No. 61-156018 is severe on errors caused in manufacturing each of the lens elements contained in each lens groups and in assembling them to complete an optical system since the respective lens groups are designed to have relatively strong refractive powers, although the total length of the whole optical system is compact.